Sampling procedure and temporal-spatial distribution of the cabbage aphid, Brevicoryne brassicae L. (Hemiptera: Aphididae), on canola.
To estimate population density of the cabbage aphid, Brevicoryne brassicae (L.), a stratified random sampling was conducted in two unsprayed canola fields in Isfahan (central Iran) during 2011-2012. Population density was determined for apterous, and alates, as well as the total population, at two plant growth phases (PGP1: From plant emergence to the end of rosette, and PGP2: From the beginning of stem elongation to ripening) on the whole plant as the sampling unit. Sources of variation in the sampling procedure were analyzed with a nested analysis of variance (NANOVA). In PGP2, aphid density in upper (10-15 cm upper part of stem) and lower (the rest of stem) parts were compared using Student's t-test. Temporal changes in spatial pattern during the growth season were evaluated using 1/k (aggregation index) and Lloyd's Patchiness Index. Results showed that differences among fields accounted for the majority of total variation observed in aphid densities and the aphids significantly preferred upper parts of canola plants. Among different indices used for analyzing spatial distribution of the aphid, Taylor's Power Law (TPL) described well the relationship between variance and mean of the population. In both PGPs spatial patterns of apterous, alates, and total population were aggregated, random, and aggregated, respectively. Estimates of 1/k and changes in the value of patchiness index revealed that the aphid population was aggregated at the beginning of rosette stage and became more dispersed with time.